Are you carrying spares around all the time or just when you go camping or something?
Three options.
1/ wind up the stock ones higher.
They will still sag abit under big loads (and if you hammer them they might break?).
2/ put type3 wagon units with late swingaxle springplates on.
This means the spring rate is about 30% better but you are limited to a 31inch tyre.
3/ buy some coilover shocks.
Easy and reasonably cheap depending on the shock you buy.
Depends how much you wanna spend or work on it.
If you are gonna rebuild the whole car at some stage and go IRS, buy a whole type 3 rear end and use only the torsion bars for the moment.